normally it takes 3-6 months to get duplicate copy, but in my case it took too much time.
My employer applied for duplicate copy in Nov-2017, and I received a week ago on 25-Jan-2019.
my I-824 was approved in Dec 2017, but USCIS didn’t send the copy because of some problem with their system.
After many followups I received.
if your application date is beyond current processing time then you can raise a “”
Outside normal Processing time" with USCIS to check ur statu.
FYI, I traveled to US with out I-824, fortunately no one asked me for approval copy at immigration.
travelling with photo copy is completely depend on ur port of entry, n ur luck.
some times at port of entry immigration person will ask you to show original.
some.time they accept photo copy n validate the authenticity of that document in their system
in my case i travelled with photo copy n no one asked me for original at POE.
this is my opinion n not a legal advice. so decide ur self.